The country's opposition has earlier called for a nationwide strike to protest Belaid's murder, and had threatened to pull out of the Constituent Assembly, which is locked in a battle to draft a new Tunisian constitution, BBC News said.

Many of the protestors chanted, "Shame, shame Shokri [sic] died," "The government should fall," and "Where is the government?" Reuters reported. In Sidi Bouzid, almost four thousand people took to the streets, where they lit tires on fire and threw stones.

The offices of the government, run by the Islamist party Ennahda, were also attacked. The Telegraph reported that demonstrators blamed Ennahda for failing to prevent the growth of fundamentalism.

Tunisian President Moncef Marzouki, traveling in France, cut short his visit and canceled a planned trip to Egypt after news of the killing.

"There are many enemies of our peaceful revolution. And they're determined to ensure it fails," Marzouki said in a speech to the European Parliament, calling Belaid a "long-standing friend" and adding that his "hateful assassination" was a threat, BBC News reported.

"This is a letter being sent to us that we will refuse to open," President Marzouki said.

Belaid was secretary general of the leftist Democratic Patriotic party. He was shot in the head and neck as he left his home.

The Associated Press cited the Tunisian government as saying Belaid was killed amid tensions over extremism. He was a critic of Tunisia's leadership, the AP wrote, targeting the dominant moderate Islamist party Ennahda.